1. Field of the Invention
The present invention relates to relatively low cost optical coatings which are formed by applying a paint or paint composition to metal surfaces to give these surfaces spectrally selective properties. A major application of such selective surfaces is for the efficient collection of radiant energy at wavelengths which are effectively absorbed by the surfaces, and more particularly as absorber surfaces in solar thermal energy collectors in order to improve the cost effectiveness of such devices.
2. Description of the Prior Art
A metal surface which has been coated with an optical absorption layer is described as being spectrally selective if the absorptance, .alpha., of the coated surface for radiation of a certain range of wavelengths is greater than the emittance, .epsilon., of the same surface for radiation of a different range of wavelengths. The ratio .alpha./.epsilon. is generally termed the selectivity of the surface; in this disclosure the terms medium selectivity and medium selective refer to .alpha./.epsilon. ratios in the range of about 2.5 to about 5; the terms high selectivity and highly selective refer to ratios .alpha./.epsilon. which are greater than about 5.
It is immediately apparent that the use of medium and highly selective surfaces in radiant energy collectors can significantly influence the efficiency of heat generation, since the absorptance for the radiation to be collected can be kept high while at the same time the heat radiation losses can be kept low. In the particular case of a solar thermal energy collector, a normal black surface may be used to absorb solar radiation efficiently, i.e., at wavelengths in the range of 0.3 to 2.5 .mu.m, approximately; however, a normal black surface, being nearly an ideal black body with a selectivity of about unity, reradiates heat energy very strongly at longer wavelengths, for example, above 2.5 .mu.m at surface temperatures in the order of 100.degree. C. On the other hand, a spectrally selective surface can be made to absorb solar radiation effectively, while reradiating much less heat energy than a normal black surface. Thus the useful heat output can be significantly increased in relation to the solar energy input, i.e., the efficiency is greater.
Various absorption coatings are known in the art which give a high selectivity to suitable metal surfaces. Such coatings are produced by wet chemical or electrochemical processes or by evaporation in vacuum or by deposition from the gas phase onto metallic substrates. In certain cases a selectivity of greater than 9 is attained. All of these coatings, however, are relatively costly because of the high investment and running costs of the necessary coating plant, so that their use in radiant energy collectors is generally not cost effective.
However, it has been shown by applicant in Solar Energy, Vol. 19, p. 263-270 (1977) that even medium selective absorber coatings can considerably increase the efficiency of radiant energy collectors, in particular flat plate solar thermal energy collectors.
In the literature paint-like coatings are described which comprise mixtures of germanium, silicon, lead sulfide, soot or black iron oxide as powder pigments with silicone resin as binder. Of these coatings, only those reported by Moore et al, Sharing the Sun, Solar Technology in the Seventies, Joint Conference USISES/Solar Energy Soc. Canada, Aug. 15-20, 1976, Winnipeg, pp. 187-204, have achieved medium selectivity. However, these coatings are stated to require a curing period of 12 hours at elevated temperature (see p. 189), a fact which rules out an economical coating of metals on a production line by this method. A further disadvantage of the coatings of Moore et al is that black iron oxide, the pigment used by them, changes to a reddish product upon heating above about 300.degree. C., thus losing its high optical absorptance. Hence, radiant thermal energy collectors using such coatings are limited to working temperatures in the range up to about 200.degree. C., allowing for a safety margin.